In the field of electronics, various electronic design automation (EDA) tools are useful for automating the process by which integrated circuits, multi-chip modules, boards, etc., are designed and manufactured. In particular, electronic design automation tools are useful in the design of standard integrated circuits, custom integrated circuits (e.g., ASICs), and in the design of custom configurations for programmable integrated circuits. Integrated circuits that may be programmable by a customer to produce a custom design for that customer include programmable logic devices (PLDs). Programmable logic devices refer to any integrated circuit that may be programmed to perform a desired function and include programmable logic arrays (PLAs), programmable array logic (PAL), field programmable gate arrays (FPGA), complex programmable logic devices (CPLDs), and a wide variety of other logic and memory devices that may be programmed. Often, such PLDs are designed and programmed by an engineer using an electronic design automation tool that takes the form of a software package.
In the course of generating a design for a PLD, programming the PLD and checking its functionality on the circuit board or in the system for which it is intended, it is important to be able to debug the PLD because a design is not always perfect the first time. Before a PLD is actually programmed with an electronic design, a simulation and/or timing analysis may be used to debug the electronic design. However, once the PLD has been programmed and is operating within a working system, it is also important to be able to debug the PLD in this real-world environment.
And although a simulation may be used to debug many aspects of a PLD, it is nearly impossible to generate a simulation that will accurately exercise all of the features of the hardware PLD on an actual circuit board operating in a complex system. For example, a simulation may not be able to provide timing characteristics that are similar to those that will actually be experienced by the hardware PLD in a running system; e.g., simulation timing signals may be closer or farther apart than what a PLD will actually experience in a real system.
In addition to the difficulties in generating a comprehensive simulation, other circuit board variables such as temperature changes, capacitance, noise, and other factors may cause intermittent failures in a PLD that are only evident when the PLD is operating within a working system. Still further, it can be difficult to generate sufficiently varied test vectors to stress the PLD design to the point where most bugs are likely to be observed. For example, a PLD malfunction can result when the PLD is presented with stimuli that the designer did not expect, and therefore did not take into account during the design and simulation of the PLD. Such malfunctions are difficult to anticipate and must be debugged in the context of the complete system. Thus, simulation of an electronic design is useful, but usually cannot debug a PLD completely.
One approach to debugging a hardware device within a working system is to use a separate piece of hardware equipment called a logic analyzer to analyze signals present on the pins of a hardware device. (For example, the HP1670A Series Logic Analyzer from Hewlett-Packard Company.) Typically, a number of probe wires are connected manually from the logic analyzer to pins of interest on the hardware device in order to monitor signals on those pins. The logic analyzer captures and stores these signals. However, the use of an external logic analyzer to monitor pins of a hardware device has certain limitations when it comes to debugging such a device. For example, such an external logic analyzer can only connect to and monitor the external pins of the hardware device. Thus, there is no way to connect to and monitor signals that are internal to the hardware device. Unfortunately, when programming a hardware device such as a PLD, it would be useful to be able to monitor some of these internal signals in order to debug the PLD.
Although some custom hardware devices may come ready made with some internal debugging hardware, this debugging hardware is typically hardwired to route specific internal signals and cannot be readily changed by an engineer who wishes to look at other signals. Also, with such built-in debugging it is not possible to choose any signal to monitor that the engineer desires, nor can triggering signals and triggering conditions be changed by the engineer. Because a PLD by its very nature is a programmable device that an engineer is attempting to program to perform a particular function, it is important to the engineer to be able to customize monitored signals, trigger signals, and trigger conditions in order to efficiently debug any particular device. Further, creating an electronic design for a PLD is an iterative process that requires creative debugging by an engineer who may wish to view almost any internal signal, and who may change his mind fairly frequently in the course of debugging a PLD within a system. Known external and internal logic analyzers do not provide this flexibility.
A further drawback to using an external logic analyzer or hardwired predetermined debugging hardware inside of a custom chip is that often the number of internal signals that an engineer desires to monitor are greater than the number of available pins on the device. For example, if there are sixteen internal signals that an engineer wishes to monitor on a device, he is unable to do this using an external logic analyzer if the device has only four pins available for debugging.
In some cases, it is possible for an engineer to employ a conventional logic analyzer to study an internal signal of a PLD. This may be accomplished by, for example, an engineer modifying his design so that a normally internal signal is routed temporarily to an output pin of the PLD. The design is then recompiled. The engineer then attaches a probe to this output pin in order to monitor the "internal" signal. Unfortunately, the engineer must recompile his design and reprogram the PLD in order to view this internal signal. Also, when debugging is complete, the engineer must again rewrite the design to remove the internal signal from the output pin, recompile the design and finally reprogram the PLD again. This can be a tedious process.
Even if an engineer is successful in routing an internal signal to an output pin of a PLD, with certain integrated circuit packages it may be extremely difficult to attach an external logic analyzer. For an integrated circuit in a dual in-line package it may be relatively straightforward to attach the probes of a logic analyzer to the top of the package as long as the package is in an easily accessible location on a circuit board. However, if the package is in a difficult to reach location because of device crowding, it may be difficult to physically attach logic analyzer probes to particular output pins of interest. Even more troublesome are integrated circuits with rows of miniature contacts located on the top of the package (e.g., "flip chips"). It is difficult to attach logic analyzer probes to particular outputs of interest with this type of package. For an integrated circuit encased in a ball grid array package with the contacts located on the bottom of the package up against the circuit board, it may be nearly impossible to attach logic analyzer probes to these small contacts located on the underside of integrated circuit package. Thus, use of an external logic analyzer has shortcomings even if an internal signal can be routed to a pin of a device.
Therefore, an apparatus and technique are desirable that would allow a logic analyzer to flexibly analyze internal signals of interest in an electronic design.